something to consider having the officiant say before the ceremony starts:

"The bride and groom would like you to be able to really enjoy their wedding day, feeling truly present and in the moment with them. They've hired amazing wedding photographers who will be capturing the way the wedding looks — and they're inviting each of you to sit back, relax, and just enjoy how the wedding feels. We respectfully ask that everyone consider leaving all cameras and cell phones off during the ceremony. Thank you."